Zombies Ate My Neighbors and Ghoul Patrol (UP)

The Flamethrower is incontrovertibly the greatest secret of all time in
Zombies Ate My Neighbors. In Level 22 [Revenge of Dr. Tongue] there is a special
Skull Door near the center of the level that you DON'T need to go through, and
is actually partially hidden (the path to it, anyway). Thus, you must've grabbed
the Skeleton Key in level fifteen or picked one up in a storage area (highly
unlikely) in order to open the door. When you eventually DO open the door, you
find a Bonus Level Icon inside. Most people assume that's what the door and the
Skeleton Key was for. But that wasn't it! You can walk into the wall to your
right and move around a bit until you hear a "bleep." Then the Flamethrower is
all yours!